The present invention relates to a novel single-station thermoforming apparatus and the method carried out by this apparatus.
The thermoforming machines falling within the scope of the invention are those machines in which a sheet-like plastics material (in general a plate or a coil bobbin) is at first heated to the plasticizing temperature thereof and then is formed or shaped by means of a suitable mold and counter-mold system.
In prior art single-station thermoforming apparatus, the heating ovens usually comprise monolithic construction panels, which are conventionally arranged both above and under the plastics material to be processed, and being provided with suitable heating means (electric resistances) arranged on the surface of each oven and oriented towards the plastics material.
The plastics material, in particular, is loaded into the heating station and is arranged herein in a horizontal position. During this loading operation, the heating ovens are withdrawn and arranged outside of the heating region, on the rear of the apparatus or on the same side thereof. Then, with the electric resistances being in a turned-on condition, the heating panels are driven towards the sheet-like plastics material. In particular, for a two-oven station arrangement, the heating ovens are designed to be "sandwich" arranged on the plastics material, thereby covering the overall surface thereof. Thus, the heating ovens are conventionally introduced into the heating station receiving at a fixed position thereof the plastics material to be processed with a parallel relationship to the sheet being processed and starting from an edge of said sheet facing the heating panels (i.e. the rear of the apparatus or the single oven access side).
This movement is continued to cause the plastics material sheet to be fully covered by the surfaces of the ovens. The thus achieved position is successively held for the time necessary to achieve a desired plasticizing temperature, and, after having achieved said plasticizing temperature, the heating ovens are driven for withdrawal in an opposite direction. With the heating panels fully disengaged or moved away from the sheet-like plastics materials, the forming step proper is started, which is usually preceded by an air blowing-in step for providing the so-called pre-stretching "ball", in order to pre-stretch the sheet plastics material. The forming operation, in particular, is performed by causing the mold or molds to approach the plastics material sheet, with a simultaneous formation of vacuum or application of pressurized air, in order to cause the plastics material to satisfactorily adhere to the mold. Upon having performed the forming operation, the molds are opened, the formed product is withdrawn from the forming station and is replaced by a new plastics material sheet to be thermoformed.
Great drawbacks are related to the above discussed thermoforming machines or apparatus, mainly with respect to the disclosed movements of the heating panel or panels. In fact, as discussed, a full covering of the plastics material sheet by the heating ovens is achieved starting from a same side (usually from the rear of the apparatus), which includes the inlet and outlet sections of the panels with respect to the heating station. This means that the plastics material portion arranged at this side is that which will receive the greatest amount of heat, whereas the opposite side (i.e. that arranged at the front of the apparatus) will be cooler than the other side.
In the above discussed prior art, great efforts have been made to overcome any thermal differentials, in particular by differently balancing the oven electric resistances. This balancing operation which, anyhow, is very complex, has not been however found as satisfactory to solve the above mentioned problem. In fact, in heating processes in which small thickness plastics material sheets are heated, even very small thermal differentials between opposite sides of the sheet material are susceptible to cause uncontrollable phenomena, such as the so-called "blistering" effect (a burning of the plastics material on the oven inlet side and a not sufficient heating on the opposite side).
As stated, the surface of the heating panel or panels included in the heating ovens must be at least equal to that of the plastics material sheet to be processed. Thus, it should be apparent that the construction of the mentioned panels will occupy a comparatively high volume and that the masses to be driven are comparatively large. Because of this reason, whereas the main surface of the heating panels corresponds to a large displacement thereof in order to cover and then disengage the surface of the plastics material, the weight of said panels will hinder and slow down the disclosed oven movements.
On the other hand, a high speed withdrawal of the panels from the heating station is indispensable in order to prevent burning or flaming, due to an accidental contact against the vary hot resistances of the bottom oven, of the material being heated. Such a risk, on the other hand, frequently occurs in monolithic construction oven thermoforming apparatus. In fact, in these prior stations, the highest heat concentration occurs at the central regions of the heating panels and, accordingly, at the central portions of the plastics material sheet covered by the heating ovens. This central portion, accordingly, will be subjected to the greatest deformation (usually a buckling deformation); and this in a region of the sheet plastics material which, since it is arranged at the greatest distance of the edges engaged in the heating station, will be most susceptible to deformation under the effect of its weight.
An inevitable consequence is that, even for small over-heatings in this central region of the plastics material sheet (which, anyhow, are rather frequent considering the discussed requirement of balancing the smaller heating occurring on portions of the sheet) may cause the material sheet to fall on the underlaying panel or oven, with a consequent burning thereof.
In order to overcome the above mentioned drawbacks, the sole solution afforded by prior thermoforming apparatus was that of quickly withdrawing the panels outside the region covered by the plastics material sheet. This operation, as stated, is however hindered by the large surfaces of the ovens and by inertial forces related to the large masses being driven. Accordingly, all of the prior thermoforming apparatus including single heating stations are subjected to frequent burning or flaming risks.
In addition to the above mentioned drawbacks, heating stations of prior art thermoforming apparatus are moreover affected by yet other drawbacks mainly related to the wiring assemblies of the heating electric resistances and the overall size of the thermoforming apparatus.
The first of these drawbacks derives from the fact that the large surface which must be covered by the electric resistances, coinciding with the surface of the oven facing the plastics material, requires to use very large length cables, and related protective assemblies for protection against heat. The second drawback, on the other hand, is directly related to the construction of prior thermoforming apparatus, usually including a "front" portion, i.e. that side of the machine through which the plastics material is loaded into the heating station, and a "rear" portion, i.e. that side of the machine where the heating panel or panels is/are held at a withdrawn or rest position. Thus, this single access operating position constitutes a typical limitation of these prior apparatus which hinders a quick and accurate installation of the apparatus in a manufacturing system.